Damn Noise

Ok i have had me carpc setup for almost a year now, and its almost perfect for my needs. I decided to run the pc through a HU recently, after installation i get this terrible engine noise. and No it was not their b4. setup : 1800xp,300w inverter blah blah blah

u can imagine the setup PC -> HU -> AMP -> SPK, it all worked fine b4 the HU. When i use AUX input ito the HU i gte the annoying noise, when i PLAY a cd through the HU (using HU CD DRIVE) i get no noise. so i thought well ill try a discman instead of the PC and of course no NOISE (engine whine). so it has to be the PC (though i didnt have this problem b4), i have good grounding on everything, amp.. HU.. inverter.

i seareched these boards and found that ppl ground their CASE, my interpretation is connecting a piece of wire to the pc case, and then grounding it.. i hope this is right. But still no reduction in noise. im stumped im even restoring to trying a ground loop isolator which ill buy soon if i sont get any better suggestions.

yeah it was grounded properly, i went and bought a $20 ground loop isolator and its cut out 99% of the noise so im happy for now, but i know its only a temp fix until i find the real problem of the cause....